1. Field of the Invention
The present invention relates to a device for propagation of electromagnetic waves through water.
2. Description of the Related Art
It has long been recognised that it would be very useful to be able to propagate electromagnetic waves efficiently in impure water, particularly seawater, for communications and other purposes, but doing so remains problematic. Conventional metal antennae (loop, dipole etc.) are rendered ineffective when placed in seawater and other impure water due to its electrical conductivity.
International patent application no. PCT/GBO1/02409, published under no. WO 01/95529 in the name of Qinetiq Ltd., describes an underwater communications system which uses submerged transmitter and receiver aerials coated in electrically insulating material. In tests of aerials of this general type, the present inventor has observed that the electromagnetic signal transmitted through water attenuates rapidly over a distance of perhaps a metre or two. At greater distances, signal attenuation with distance becomes much less rapid, and a detectable signal penetrates a much longer distance into the surrounding seawater, but its power represents only a small fraction of the power input to the antenna. FIG. 1 is a graph of the relevant experimental data. Rapid signal attenuation with distance is seen at short range. At a distance of just 1-2 metres, the signal is weakened by roughly 100 decibels. However a detectable (albeit weak) signal is observed at much greater distances of 90 metres and more, and the attenuation with distance at these larger distances is far less rapid.